This application claims the priority of Korean Patent Application No. 2003-29073, filed on May 7, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entity.
1. Field of the Invention
The present invention relates to a technique for forming a high quality ohmic contact with improved current injection and spreading capabilities, as well as low ohmic contact resistance and high transmittance. The technique for forming a high quality ohmic contact is one of the key techniques for the fabrication of short-wavelength light emitting diodes (LEDs) emitting blue and green visible light and ultraviolet light, and laser diodes (LDs) using a gallium nitride (GaN) semiconductor.
2. Description of the Related Art
The formation of a high quality ohmic contact between a semiconductor and an electrode is very important to manufacture photo-related devices using a gallium nitride semiconductor such as light emitting diodes and laser diodes.
Hitherto, in the case of p-type gallium nitride, thin film electrodes based on nickel (Ni), e.g., a thin metal film of nickel/gold (Ni/Au), have been widely used as thin film electrode structures for ohmic contact. Recently, a variety of metal systems for ohmic contact such as Ni/Pt/Au, Pt/Ni/Au, Pd/Au, etc., are being developed.
However, there are some problems in the formation of a high quality ohmic contact using p-type gallium nitride. Among these problems, hydrogen atoms (H) of ammonia (NH3), which is an atmospheric gas used for growing p-type gallium nitride, bind with magnesium (Mg) as a p-type dopant to form a Mg—H complex exhibiting electric insulating properties, thereby exhibiting a low effective carrier concentration and a high surface resistance. Accordingly, the formation of a high quality ohmic contact is difficult. Moreover, a low carrier (hole) injection makes the manufacture of high grade optical devices difficult.
One example for increasing the doping concentration of p-type gallium nitride is disclosed in Japanese Patent Laid-open No. 2001-35796. According to this publication, gallium nitride having an increased carrier concentration is obtained by bring a metal, e.g., zirconium (Zr), having a high hydrogen affinity into contact with the gallium nitride, followed by activating a p-type dopant present in a crystal of the gallium nitride through annealing. As another example, nickel, gold-zinc alloy and gold (Ni/Au—Zn/Au) are sequentially deposited on a gallium nitride layer to form an ohmic contact.
Another problem is that no high quality ohmic contact systems having a high transmittance in the short-wavelength region have been developed. A nickel/gold (Ni/Au) system currently employed in the LED process has a very thin thickness (<˜100 Å) and a transmittance of 80% or more. The ohmic contact using such a thin metal layer can achieve high transmittance, but it is difficult to obtain a sufficient current injection and spreading capabilities, causing low efficiency of optical devices.